Process for producing phase-stabilized ammonium nitrate

ABSTRACT

A method for producing phase-stabilized particulate ammonium nitrate (AN) through integration of a diamine complex of the metals copper, nickel or zinc into the crystal lattice of AN, wherein AN is reacted with inorganic CU, Ni and/or Zn compounds, is characterized in that carbonates and/or hydroxide carbonates and/or hydroxides are added as inorganic compounds of the metals Cu, Ni and/or Zn. Moreover, additives are added to reduce the melting temperature of AN.

BACKGROUND OF THE INVENTION

The invention concerns a method for the production of phase-stabilizedparticulate ammonium nitrate (AN) by incorporating a diamine complex ofthe metals copper, nickel or zinc into the crystal lattice of the AN,wherein the AN is melted and reacted with inorganic copper, nickeland/or zinc compounds.

Ammonium nitrate is used as oxidant in propellants and explosivesubstances, in gas generators, in rockets and recently also in air bags.While AN disintegrates with slow heating at temperatures above itsmelting point of 169.5° C. into water and dinitrogen monoxide,disintegration at higher temperatures or under rapid heating(exothermal) can occur as a detonation thereby releasing oxygen,nitrogen and nitric oxides.

Pure AN has five different crystal modifications depending on thetemperature, i.e. cubical in the temperature interval between 125° C.and its melting point of 169.5° C. (modification 1), tetragonal in thetemperature interval between 84° C. and 125° C. with a density ofapproximately 1.67 g/cm³ (modification II), orthorhombic in thetemperature interval between 32° C. and 84° C. with a density ofapproximately 1.66 g/cm³ (modification III), also orthorhombic between−18° C. and 32° C. (modification IV), but with a density ofapproximately 1.73 g/cm³ which represents a density change ofapproximately 4% compared with the density of modification III, andorthorhombic pseudotetragonal (modification V) at temperatures below−18° C. Modifications II and V are thereby very similar with respect totheir density and their lattice structure and provide almost identicaldiagrams in X-ray diffraction measurements.

In particular, the density difference between the modifications III andIV during heating of AN to temperatures above 32° C. produces tensionand the formation of cracks in the structure of the formed chargescontaining AN as oxidant.

Different types of additives have been proposed to phase-stabilize AN.It is e.g. known to stabilize modification III in the temperatureinterval between −20° C. and 100° C. through the addition of alkalinenitrates, such as potassium nitrate. The AN particles mixed withnitrates, however, tend to bake together and thereby exhibit poor flowand are difficult to homogenize in mixtures (U.S. Pat. No. 3,018,164).Moreover, modification III is not sufficiently stabilized through theaddition of potassium nitrate, in particular when AN, mixed withpotassium nitrate, cools down to temperatures below approximately −30°C.

Phase stabilization, which was successful in practice, of the similarmodifications II and V and modification IV in a temperature interval ofapproximately −100° C. to 100° C. is achieved by adding metal ammoniates(metal ammine complexes), preferably via ammine complexes of the metalscopper, nickel and zinc (DE 17 67 757 A1, DE 21 25 755, EP 0 405 272B1).

Production of metal ammine complexes or integration in the crystallattice of AN is effected e.g. through melting a mixture of AN and metaloxide (DE 21 25 755 C3). AN is thereby mixed with up to 10% of the metaloxide, is melted and the molten mass is transferred into a solid state.This method has the disadvantage that the ammine complex isinsufficiently integrated into the AN lattice since the reaction betweenmetal oxide and AN is very slow and the ammine complex disintegrates atan increased temperature, i.e. the melting temperature of ammoniumnitrate. The formation frequency and simultaneous disintegration thuscompete with one another and residues of the non-reacted metal oxidesremain in the phase-stabilized product which have a negative influenceon the combustion of ammonium nitrate.

EP 0 405 272 B1 discloses a method in which the diamine complex isproduced through reaction of the metal oxide with AN in a solid-statereaction at 110° C. to 170° C. This method is also demanding andexpensive since the diffusion rate of the solid-state reaction is lowand there are residues of non-reacted metal oxides in the AN. Both leadto an inhomogeneous product. DE 36 42 850 C1 describes a method whereinmelted AN is reacted with copper and/or nickel oxide thereby forming therespective diamine complexes (equations 1 and 2).

2NH₄NO₃+CuO→[Cu(NH₃)₂]²⁺+2NO₃ ⁻+H₂O  (1)

2NH₄NO₃+NiO→[Ni(NH₃)₂]²⁺+2NO₃ ⁻+H₂O  (2)

The molten mass must be prepared in small spatially separated chargesfor safety reasons to be subsequently processed in a spraying deviceinto spherical phase-stabilized AN particles.

According to U.S. Pat. No. 5,071,630 A1, zinc oxide is used for formingthe diamine complex instead of copper and/or nickel oxide. Zinc oxide isadded to AN in the molten phase, the molten mass is dried with inert gasand charged with ammonia to replace the NH₃, discharged during drying.Subsequently, the molten mass is optionally dissolved in pure ammoniumnitrate and the obtained product is cooled and granulated.

The metal oxides (CuO, NiO and ZnO) are thereby also disadvantageoussince they react only slowly with AN and the formed diamine complexstarts to disintegrate at temperatures above the melting temperature ofAN. Small amounts of non-reacted nickel oxide or copper oxide thereforealways remain in the product which have a negative influence on theburning behavior of AN. The metal oxides deposit in the reactioncontainer and the production plant must be frequently cleaned to preventtransport of the deposited oxides into the subsequent charge. Thisincreases the cost of the method. Moreover, contaminations, inparticular, toxic nickel oxide cannot be excluded.

It is the underlying purpose of the invention to improve the knownmethod by reducing the reaction time required for forming the diaminecomplex, preventing soiling of the product through non-reacted metaloxides, improving the quality of the product and reducing productioncosts.

SUMMARY OF THE INVENTION

In accordance with the invention, this object is achieved with theabove-mentioned method in that carbonates and/or hydroxide carbonatesand/or hydroxides and/or hydroxide nitrates are added as inorganiccompounds of the metals copper, nickel and/or zinc.

The inventive compounds, having considerably less toxic substances thanthose of the corresponding oxides, achieve a reaction of almost 100%with considerably less reaction time such that there are no residues ofthe compounds in the product and in the reaction container. A highlypure phase-stabilized AN with exact specification is obtained.

DESCRIPTION OF THE PREFERRED EMBODIMENT

While the diamine complex of nickel (II) stabilizes the modification IVof AN, the diamine complexes of copper (II) or zinc (II) stabilize themodifications II or V. In any case, the modification of AN stabilized inthis fashion is stable within a large temperature range of approximately−100° C. to 70° C.

The large reaction yield achieved through adding copper, nickel or zinccarbonates, hydroxide carbonates, hydroxides, or hydroxide nitrates inaccordance with the invention is based on the special reaction behaviorof the carbonate or hydroxide anions and on the reduced stabilitycompared to the corresponding metal oxides due to reduced latticeenergy. The carbonate or hydroxide anions react spontaneously with theammonium ion of AN thereby releasing ammonia (equations 3 and 4).

OH⁻+NH₄ ⁺→NH₃+H₂O  (3)

CO₃ ²⁻+2NH₄ ⁺→2NH₃+CO₂  (4)

The produced ammonia reacts again with the respective cations of themetals Cu, Ni or Zn thereby forming the diamine complex.

The gross reactions of the hydroxides, carbonates and hydroxidecarbonates with AN are exemplarily summarized in equations 5 through 7for the copper cation.

2NH₄NO₃+Cu(OH)₂→[Cu(NH₃)₂]²⁺+2NO₃ ⁻+2H₂O  (5)

2NH₄NO₃+CuCO₃→[Cu(NH₃)₂]²⁺+2NO₃ ⁻+CO₂+H₂O  (6)

4NH₄NO₃+Cu(OH)₂.CuCO₃→2[Cu(NH₃)₂]²⁺+4NO₃ ⁻⁺CO₂+3H₂O  (7)

Since copper carbonate (equation 6) is difficult to obtain in its pureform, the use of double salts such as CuCO₃.K₂CO₃ is possible. Insteadof pure hydroxides, hydroxide nitrates such as 3Cu(OH)₂.Cu(NO₃)₂,Ni(OH)₂.Ni(NO₃)₂.6H₂O or 4Zn(OH)₂.Zn(NO₃).2H₂O can also be added. AN isthereby not soiled with a foreign anion.

A further preferred embodiment provides that when the reaction of ANwith the inventive additives for phase stabilization shall take place inthe liquid phase, additives are added for reducing the meltingtemperature of AN to reduce the danger of explosion and the heating uptime. The process may then be carried out in one single, optionally alsolarge-volume reaction container such as e.g. heatable kneading devices,screw mixers or revolving cylindrical furnaces and must not be carriedout, for safety reasons, in several small charges in parallel or onebehind the other. The charge change can be considerably acceleratedsince the plant components do not have to be cleaned from soilingsediments. The inventive method can even be carried out continuouslywhen corresponding technology is used for the system.

While in a preferred variant, additives can be added which reduce themelting point of AN according to Raoults' laws, it is possible toprovide in particular such additives which form, in small concentrationsin the AN molten mass, an eutectic mixture, to utilize a melting pointin the eutectic range which is lower than that of the pure substances.

Preferably, water and/or metal nitrates are added. Water has theadvantage that it volatilizes e.g. during spraying in a cooled gas sinkflow. The advantage of metal nitrates is based on the fact that theammonia nitrate is only soiled by the cation of the added metal nitrateif a different cation is selected than the cations copper, nickel orzinc added for phase-stabilization of AN in the form of carbonates,hydroxide carbonates and/or hydroxides. For this reason, in particularcopper, nickel or zinc nitrate is used. If the quality of thephase-stabilized AN is not impaired by the presence of further cations,other nitrates, e.g. alkaline or alkaline earth nitrates such as NaNO₃,Ca(NO₃)₂ or Mg(NO₃)₂ can also be added. While the hydroxides, carbonatesor hydroxide carbonates of the metals copper, nickel or zinc are addedin an amount corresponding to the desired portion of the diamine complexin the product, the amount of the substances lowering the melting pointmay, optionally, be small. The proportion of the diamine complex withphase-stabilized AN for use in gas generators of rockets is 2 to 5 mass%, for use as propellant for air bags up to 50 mass %.

The invention is described below by means of examples:

EXAMPLE 1

48.5 kg of AN and 2.1 kg of Cu(OH)₂.CuCO₃ are mixed and melted in amelting vessel. When the molten mass has reached a temperature of 180°C., it is immediately sprayed in a cooling gas sink flow. The moltendrops solidify into round parts which contain the diamine nickel (II)complex integrated in the ammonia nitrate lattice for phasestabilization. Depending on the setting of the spraying parameters, oneobtains average grain sizes between 20 and 300 μm. The product, to whichoptionally up to 1% auxiliary substances can be added, such asanti-baking means, is used for producing solid propellants.

The quality of the ammonia nitrate phase-stabilized in this fashion wasexamined through wet chemical trace analysis for non-reacted copperhydroxide carbonate, which could not be found.

EXAMPLE 2

38.8 kg of AN and 2.0 kg of nickel hydroxide carbonateNiCO₃.2Ni(OH)₂.4H₂O are mixed and heated in a vessel. To reduce themelting point, 1.2 l water is added corresponding to 3 weight %. Alreadyat 120 to 130° C., the molten mass heats up considerably faster due toimproved thermal transfer onto the molten material. During furtherheating with continuous stirring, a considerable part of the added waterescapes when the vessel is open. The remaining water is removed in aspraying process according to example 1, wherein comparable grain sizesare obtained. Approximately 0.5% anti-baking agent is added to theproduct which is then used for producing solid propellants. Also in thiscase, wet chemical examination did not show any residues of non-reactednickel hydroxide carbonate in the product.

EXAMPLE 3

For producing a product according to example 2, only 19.4 kg of AN and2.0 kg of nickel hydroxide carbonate and 1.2 l water are heated in avessel. Due to the small AN proportion, the mixture already melts atapproximately 90° C. such that heating up is considerably accelerated byearly switching on of the mixer. During further heating up, another 19.4kg of AN is added thereby keeping the molten mass in a stirrable state.When the temperature has reached approximately 170°-180° C., the moltenmass is sprayed. Gradual addition of AN achieves early melting and thusfaster heating thereby permitting increase of the charges by up to 40%and therefore substantially reducing method costs.

EXAMPLE 4

38.8 kg of AN and 2.0 kg of nickel hydroxide carbonate NiCO₃.2Ni(OH)₂are mixed and heated in a vessel. To reduce the melting point, 0.2 kg ofnickel nitrate hexahydrate is added. Already at 130° C. a stirrablemolten mass is obtained which heats up more quickly due to improved heattransfer to the molten material. During further heating under continuousstirring, an essential part of the water added along with the nickelcompounds escapes when the vessel is open. The rest of the water isremoved in a spraying process according to example 1 wherein comparablegrain sizes of the phase-stabilized AN are obtained. Approximately 0.5%anti-baking agent is added to the product which is then used forproducing solid propellants. No traces of nickel hydroxide carbonatecould be found.

EXAMPLE 5

95 g of AN is heated and melted e.g. in an oil bath to approximately170°-180° C. 6.9 g zinc hydroxide carbonate Zn(OH)₂.ZnCO₃ were added tothe molten mass under continuous stirring. The added zinc saltcompletely reacts within a few seconds to form a clear molten mass. Themixture is poured into a mortar and ground while it is still hot.

EXAMPLE 6

3.2 kg of AN is reacted with 1.1 kg of Cu(OH)₂.CuCO₃ in a heatablekneading device at approximately 140° C. for producing aphase-stabilized ammonium nitrate having a high proportion of diaminecomplex, e.g. for use in gas generators or air bags. With this massproportion, the mixture contains, after completed reaction, the samemolar amounts of AN and diamine copper (II). To reduce the meltingtemperature, 100 g of Cu(NO₃)₂.3H₂O is added. After 30 minutes, thetemperature is reduced and the molten mass solidifies into a compacteasy to handle product during operation of the kneading device.

The product was examined for non-reacted products of copper hydroxidecarbonate using wet chemical and X-ray spectroscopic means. Even withsuch high additions, no negative result was obtained.

What is claimed is:
 1. A method for producing phase-stabilizedparticulate ammonium nitrate through integration of a diamine complexinto a crystal lattice of ammonium nitrate, the method comprising thesteps of: a) melting the ammonium nitrate; and b) reacting, followingstep a), the ammonium nitrate with at least one inorganic compoundselected from the group consisting of copper carbonate, copper hydroxidecarbonate, copper hydroxide, copper hydroxide nitrate, nickel carbonate,nickel hydroxide carbonate, nickel hydroxide, nickel hydroxide nitrate,zinc carbonate, zinc hydroxide carbonate, zinc hydroxide, and zinchydroxide nitrate.
 2. The method of claim 1, wherein said inorganiccompound comprises at least one bivalent carbonate.
 3. The method ofclaim 2, wherein said at least one bivalent carbonate is selected fromthe group consisting of CuCO₃, NiCO₃ and ZnCO₃.
 4. The method of claim1, wherein said inorganic compound comprises at least one bivalenthydroxide carbonate.
 5. The method of claim 4, wherein said at least onebivalent hydroxide carbonate is selected from the group consisting ofCu(OH)₂.CuCO₃, 2Ni(OH)₂.NiCO₃.4H₂O, and Zn(OH)₂.ZnCO₃.
 6. The method ofclaim 1, wherein said inorganic compound comprises at least one bivalenthydroxide.
 7. The method of claim 6, wherein said at least one bivalenthydroxide is selected from the group consisting of Cu(OH)₂, Ni(OH)₂, andZn(OH)₂.
 8. The method of claim 6, wherein said inorganic compoundcomprises at least one bivalent hydroxide nitrate.
 9. The method ofclaim 8, wherein said at least one bivalent hydroxide nitrate isselected from the group consisting of Cu(OH)₂.Cu(NO₃)₂,Ni(OH)₂.Ni(NO₃)₂.6H₂O, and 4Zn(OH)₂.Zn(NO₃)₂.2H₂O.
 10. The method ofclaim 1, further comprising the inclusion of additive for reducing amelting temperature of the ammonium nitrate.
 11. The method of claim 10,wherein said additive is water.
 12. The method of claim 10, wherein saidadditive is at least one metal nitrate.
 13. The method of claim 12,wherein said at least one metal nitrate is selected from the groupconsisting of copper nitrate, nickel nitrate, and zinc nitrate.